Implements on work machines are commonly operated through the use of hydraulics. Control valves play an important role in controlling the flow and pressure of the hydraulic fluid as it is distributed to the implements or other work elements and/or attachments associated with a particular work machine. Such valves can be controlled in a number of ways. They can be controlled mechanically using pilot pressure for hydraulic activation such that the valve can either provide constant flow or constant pressure, or with the increasing demand for electrohydraulics, such valves can also be controlled via electronic solenoids or other electronic actuator means either with or without feedback, depending upon the requirements of the application.
To achieve open-loop control of such valves, actuators without feedback are used. However, some applications require more accuracy, less hysteresis, better repeatability, fast response and greater power capacity. To meet these requirements, a closed-loop control with feedback is required. Most available closed-loop feedback control systems presently on the market include either spool position feedback or pressure feedback. When spool position feedback is used, a constant flow rate can be achieved. When pressure feedback is used, a constant pressure can be achieved.
There are two types of electrohydraulic valve designs often used to control the operation of a wide variety of different types of implements used on a wide variety of different types of work machines such as front end loaders, backhoe loaders, dozers and other earthmoving and construction equipment, namely, an open center valve and a closed center valve. The slot designs of the spools of each valve, which are quite complex, dictate their performance characteristics. An open center valve uses the setting of the spool position to provide constant flow, regardless of load, which in turn provides the implement or work element with a constant speed of movement. Such valves are relatively inexpensive and, more importantly, are load pressure sensitive so that the operator can learn to "feel" the pressure being exerted against the implement or its actuating cylinder and thus better control the operation and movement of the implement. However, open center valves cannot provide a constant flow at high pressure. In addition, such valves are associated with high power losses and are thus inefficient, especially for heavy loads operating at low speeds. A closed center valve, on the other hand, provides only the flow required to meet the implement demand and operates with a fixed pressure margin above the highest system load. These "constant pressure" valves are typically used in slow speed, high load applications. As a result, this type of valve is more efficient and more compatible with closed-loop control performance characteristics as compared to open center valves. However, such closed center valves are characterized by low damping and thus lack the pressure control of open center valves.
Implements are used in a wide variety of different applications which require the performance characteristics of both open center valves (in particular, pressure control) and closed center valves (in particular, flow control). For example, in a backhoe loader application when the shovel is digging, a low fluid flow rate and high pressure to the implement (shovel) is normally required. On the other hand, when the shovel is moved upwardly and rotated to dump the material at a new location, a high flow rate and low pressure to the implement (shovel) is normally required. With only constant flow control as provided by closed center valves, if the shovel happens to hit an underground pipe or other obstruction, the shovel will continue to move thereby breaking the pipe or other object due to the lack of pressure control which is provided through the use of open center valves. However, with only constant pressure control as provided by open center valves, the shovel will stop digging if a pipe or other obstruction is encountered, but the speed of movement of the shovel will be reduced with a full shovel as compared to an empty shovel due to the lack of flow control provided by closed center valves.
Therefore, a desired implement control system should have pressure and flow control flexibility such that both the flow and pressure can be simultaneously controlled. Moreover, such control should be software-controlled so as not to depend upon the specific valve design used. By controlling both flow and pressure simultaneously with a single valve design, the performance of hydraulic machine implements including their associated actuating mechanisms such as actuating cylinders, motors and the like over a variety of different applications can be optimized.
Accordingly, the present invention is directed to overcoming one or more of the problems as set forth above.